A chain saw has been conventionally known as a portable work machine driven by an engine. Such a chain saw sometimes includes a top handle provided above a body that houses the engine (for example, see Patent Document 1). While the top handle is commonly used for a relatively small chain saw, a carburetor and an air cleaner are sometimes accommodated in a rear portion of the top handle to further downsize the chain saw.    Patent Document 1: JP-A-5-195891